Only one instance of a contact can be in a segment and campaign at the same time.
For example, if you created 10 shared lists each containing only your email address and added them to the same segment, the final count and list of contacts in that segment would be your one email address. If you then sent an email to that segment, it would only send one email to that one email address.
Essentially, segments automatically dedupe and count each list within it for you.
